1. Field of the Invention
The present invention relates to a laser apparatus.
2. Description of Related Art
A wavelength of a solid laser is used as a standard for length, the wavelength of the solid laser being in a 532 nm region in continuous-wave oscillation using a Nd:YAG crystal excited by a semiconductor laser, for example, as a gain medium. In effect, in order to perform length measurement using the wavelength of a laser light, the laser light must oscillate at a single frequency, i.e., at a single longitudinal mode. Moreover, in order to stabilize the frequency of the laser light using a spectroscopic technique for atom or molecule absorption lines, it must be possible to select an oscillation frequency as desired. For example, in an iodine stabilized laser using a spectroscopic technique for iodine molecule absorption lines, a laser light with a high degree of frequency stability can be obtained by controlling an oscillation frequency to a center of a saturated absorption signal (Japanese Patent Laid-open Publication No. 2008-130848).
A principle of frequency search using a differential signal of a light output signal that contains a saturated absorption signal of an iodine molecule is already known (Ishikawa, Jun, “Portable national length standards designed and constructed using commercially available parts,” Synthesiology, National Institute of Advanced Industrial Science and Technology, November 2009, Vol. 2, No. 4, pp. 276-287).
However, the inventor of the present invention has discovered an issue, discussed below, with respect to the above-noted technique. In the above-noted technique, a drive current is monitored and degradation of the laser over time is detected. However, the laser apparatus may be exposed to an external disturbance such as vibration from a surrounding operating environment. For a laser apparatus for which a high degree of frequency stability is demanded, exposure to an external disturbance may give rise to an event where the frequency of laser light temporarily deviates from a predetermined frequency and the laser apparatus can no longer meet the required level of frequency stability. For example, when the laser light is used for length measurement or the like in a state where the required level of frequency stability cannot be met, reliable measurement results cannot be obtained. Accordingly, detection of an event where the frequency of the laser light temporarily deviates from the predetermined frequency is desired. However, the above-noted technique merely monitors the drive current of the laser, and therefore detection of an event where the frequency of the laser light temporarily deviates from the predetermined frequency is not possible.